(1) Field of the Invention
The present invention relates to a liquid scintillator solution which consists essentially of a polyalkylene glycol derivative of the formula: ##STR2## wherein R.sup.1 to R.sup.5 are each a hydrogen atom, a straight chain or branched chain alkyl group of 1 to 10 carbon atoms or a cyclohexyl group, R.sup.6 and R.sup.7 are each a hydrogen atom or a methyl group provided that both R.sup.6 and R.sup.7 are not methyl groups, m is 0 or 1 and n is a value between 2 and 15; and a scintillator.
(2) Description of the Prior Art
Liquid scintillator solutions (or cocktails) are used in liquid scintillation counting. Liquid scintillation counting is extensively employed for measuring radioactivity including low energy .beta.-radiation, for example, for measuring radioisotopes such as .sup.14 C, .sup.35 S, .sup.3 H, etc. In general, in liquid scintillation counting, a sample for counting is first prepared by adding a radioactive sample to a liquid scintillator solution and dissolving, dispersing or emulsifying it.
The components of a liquid scintillator solution are a solvent and one or two scintillators. Conventionally, the solvent is usually an aromatic liquid such as an alkylbenzene like toluene. The function of this solvent is to absorb energy generated by the radioactive decay as excitation energy and then transfer it to the scintillator. The scintillator will, in turn, convert the transferred energy to light. Although aromatic liquids such as alkylbenzenes have good energy transfer efficiency, their main disadvantage is that they cannot be used with a water soluble sample.
Most samples of biological and environmental interest are aqueous solutions and contain salts, proteins, carbohydrates etc. Most of these samples are insoluble in aromatic hydrocarbons and, accordingly an accurate and reproducible measurement cannot be expected with these solvent systems. This is because the average range of a .beta.-ray from .sup.3 H in water is 1.2 microns and that from .sup.14 C is 12 microns and hence the scintillation does not take place unless the scintillator particles are present within such short range.
In order to overcome such problem, two methods have been developed (see Liquid Scintillation Measurements, by Hiroaki Ishikawa, published by Nan-zan Do, 1977, p. 22). One of these methods employs a co-solvent which is soluble in both water and aromatic solvents, for example, alcohols, dioxane etc. By this method, an accurate and reproducible measurement can be attained if the amount of sample is small enough to give a uniform solution. However, with an increase in the amount of the sample, it becomes more difficult to obtain a homogeneous solution. Therefore, in this method, the amount of the sample is restricted and the components measured, such as salts, proteins, carbohydrates, can only be used within a limited concentration range. Furthermore, dioxane is volatile and its vapor is toxic.
The second method is to prepare a liquid scintillator solution by adding a surfactant or an emulsifier to an aromatic hydrocarbon. Thus, this method provides a virtually uniform system in which numerous appropriately fine micelles are present as a colloidal suspension, dispersion or emulsion. Although this method can measure radioactivity efficiently over a range where such uniform gel is present, there are disadvantages. There is a restriction on the amount of a sample which can be dissolved in the liquid scintillator solution and it is difficult to predict the influence of temperature on the solubility and dispersibility of the sample. Another disadvantage is difficulty in predicting the influence of the time factor on the stability on the sample. Still another disadvantage is that when the amount of a water soluble sample added is increased, the viscosity of a liquid scintillator solution of this type suddenly increases beyond the point of gel formation (see Comparative Examples below). In an ordinary batchwise measurement, such as increase in viscosity prevents the formation of a gel having a uniform composition. A nonuniform dispersion results in a variation in activity from one portion of the test vial to the next. In a flow type liquid scintillation counting, e.g., liquid radiochromatography, such liquid scintillator solutions are only useful to an extremely limited extent due to their inability to pass uniformly through the flow tube.
In addition, all liquid scintillator solutions which have hitherto been employed contain, as the chief components, volatile flammable solvents, which is a great drawback. Still further, these liquid scintillator solutions cannot be disposed of merely by diluting with a large amount of water and discarding, because copious insolubles would be produced, a chief reason for the difficulty in disposal of the used liquid scintillator solutions.